Assymetric hollow fiber heat exchangers

ABSTRACT

A method of making an asymmetric polymeric hollow fiber includes dissolving a polymer in one or more solvents to form a polymer solution, providing the polymer solution to a spinneret or device in which a bore fluid is directed to flow through a first tube surrounded by a larger, second tube through which the polymer solution flows, and co-extruding the polymer solution and bore fluid. The method forms a first, substantially impermeable portion of the asymmetric polymeric hollow fiber and second, porous portion of the asymmetric polymeric hollow fiber.

CROSS REFERENCE TO RELATED PATENT APPLICATION

The present application claims benefit of priority of U.S. Provisional Patent Application Ser. No. 61/944,022, filed on Feb. 24, 2014 which is incorporated herein by reference in its entirety.

FIELD OF THE TECHNOLOGY

One or more aspects relate to polymer hollow fibers (HF) used for heat transfer, their manufacture, and their use. Uses for these membranes include, but are not limited to, heat exchange (HX) processes in which the primary transport is of thermal energy. Environments with relatively low pressure and temperature requirements, in which one or more of the fluids is saline, are particularly appropriate.

BACKGROUND

A number of currently practiced and emerging water treatment and power production processes require the ability to transfer heat between one or more fluid streams; often in environments that are highly corrosive, such as with concentrated brine streams involved in the desalination of seawater, groundwater, or oil and gas produced water streams; or in heat transfer fluids, such as absorption heat pump salt solutions such as LiBr, or geothermal brines. A common set of requirements for such energy transfer devices is that they be mechanically robust, resistant to corrosion, inexpensive, and compact. Ideally these devices will allow for relatively high heat transfer rates and reasonably high fluid flow rates without excessive pressure drop.

SUMMARY

Embodiments herein describe methods of fabrication, characteristics of composition, and processes for use of asymmetric polymer membranes for heat exchange. Methods include, but are not limited to: casting an asymmetric heat exchange hollow fiber with a dense top layer and porous substructure in a dry/wet fabrication process; casting an asymmetric HX hollow fiber through a thermally enhanced wet fabrication process; forming a HX module with asymmetric polymeric hollow fibers of the type disclosed herein to allow for the exchange of heat between two streams; causing mineral scale to form in the porous structure of a hollow fiber, and/or on its surface, before or after placing the fiber in a module. An asymmetric hollow fiber heat exchanger for heat exchange between fluids, the fiber being dense at its surface and porous in its substructure. Non-polymeric material may be located within pores and/or a surface of the heat exchanger. Transfer of heat between streams occurs by means of a heat exchanger composed of asymmetric polymeric hollow fibers, with or without non-polymeric material within the pores of the fibers and/or on its surface. Heat exchange fibers of the type described above may be further coated with a material intended to protect the material added to the pores of the fiber and/or to its surface.

An embodiment relates to a method of making an asymmetric polymeric hollow fiber including dissolving a polymer in one or more solvents to form a polymer solution, providing the polymer solution to a spinneret or device in which a bore fluid is directed to flow through a first tube surrounded by a larger, second tube through which the polymer solution flows, co-extruding the polymer solution and bore fluid, forming a first, substantially impermeable portion of the asymmetric polymeric hollow fiber and forming a second, porous portion of the asymmetric polymeric hollow fiber.

Another embodiment relates to a method of using an asymmetric polymeric hollow fiber device having at least one asymmetric polymeric hollow fiber, the method including providing a first fluid having a first temperature to an inner surface of a wall of the asymmetric polymeric hollow fiber, providing a second fluid having a second temperature to an outer surface of the wall of the asymmetric polymeric hollow fiber, wherein the second temperature is different from the first temperature and transferring heat from the first fluid to the second fluid or the second fluid to the first fluid through the wall of the asymmetric polymeric hollow fiber.

Another embodiment relates to an asymmetric polymeric hollow fiber including a first, substantially impermeable portion and second, porous portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an asymmetric polymeric hollow fiber according to an embodiment.

FIG. 2 is a schematic side view of an asymmetric polymeric hollow fiber device according to an embodiment.

FIG. 3 is a schematic perspective view of a method of using an asymmetric polymeric hollow fiber according to an embodiment.

DETAILED DESCRIPTION

As discussed in more detail below, an embodiment provides polymer interfaces that are thin enough to optimize: the heat transfer performance characteristics of the polymer, in relation to thickness-dependent transport properties, such as thermal conductivity; the mass and energy transport effects at the boundaries of such surfaces; and material properties that provide sufficient mechanical and chemical robustness for long term use. Typically the mechanical strength characteristics of polymer heat exchangers are met by the thermal extrusion of relatively thick dense polymer tubes, reducing their heat transfer performance and increasing their cost. The polymers commonly used for heat transfer are typically much more expensive than polymers used for common water separation applications. For example, high temperature fully imidized polyimide, PEEK, or PTFE can cost over 10 times more by weight than simple membrane structural polymers such as polysulfone or cellulose acetate. Embodiments discussed below reduce the quantity of polymer necessary for the construction of HX fibers without significantly compromising on their heat transfer and strength characteristics.

The thinness of the material used for heat exchange processes is often significant, in addition to its inherent heat transfer characteristics. For example, the thermal conductivity of a material is inversely proportional to the thickness of that material—a thin polymer with a relatively low intrinsic thermal conductivity can be a more effective conductor than a relatively thick plate of metal. Conversely, the absence of material, such as pores in an asymmetric fiber, when filled by an air gap, can be a highly effective insulator, and should preferably be avoided. Similarly, if such pores are filled by a material with a higher conductivity than the polymer, the performance of the fiber may be enhanced.

One such example of the material thickness relative to conductivity may be found in the properties of a film of polyimide compared to that of a plate stainless steel. Polyimide has an intrinsic thermal conductivity of 0.52 Watts per meter of thickness (over a 1 m² unit area) per degree Kelvin difference in temperature, or 0.52 WmK. In contrast, stainless steel has a thermal conductivity over 30 times higher, at 16 WmK. However, metal heat exchangers typically have tube thicknesses of not substantially less than 1 mm to allow for sufficient strength and corrosion resistance. Polymer films may be made very thin, for example as described in more detail below, such that one surface of the hollow fiber is dense and without pores, but the remainder of the fiber is porous, with only a percentage of the space filled by polymer. The dense portion of the fiber may be between approximately 1-100 microns in thickness, with the overall fiber being between approximately 25-250 microns thick.

By way of comparison with metal heat exchangers, one may assume a stainless steel tube thickness of 1 mm, which allows one to calculate a heat transfer coefficient for the tube of 16,000 Wm²K. Assuming a polyimide fiber thickness of 50 microns, or 0.05 mm, the heat transfer coefficient for the fiber is 10,400 Wm²K. This would seem to still leave the polymer at a disadvantage, but one should also consider the overall heat transfer coefficient of a heat exchange surface, which in many cases is dominated by the resistance to heat transfer of the boundary layers at the interface of the heat transfer material and fluids (the greatest resistance in a heat transfer process largely determines the overall rate). For example, a stainless steel heat exchanger transferring heat between steam and forced convection water is expected in practical use to have an overall heat transfer coefficient of typically not more than 1,000 Wm²K, substantially below the ideal coefficient for stainless steel without boundary layer resistances (16,000 Wm²K for 1 mm thickness). In such an environment, the practical difference between a 1 mm thick stainless steel heat exchanger and one made of 0.05 mm thick polyimide may be expected to be minimal.

In fact, boundary layer resistances in heat transfer may in many cases, particularly in water/water heat transfer applications, to allow for equivalent performance between, for example, a 1 mm wall thickness stainless steel tube and a polymer hollow fiber with a wall thickness of as much as 500 microns (0.5 mm) Even in applications in which the polymer conducts less heat on an area basis than a metal heat exchanger, differences in the dimensions of a HFHX and a metal heat exchanger may favor the polymer HX. For example, in both metal shell and tube and polymer HF heat exchangers, water flowing inside the lumen of the fiber or tube is most likely laminar, leading to reduced heat transfer. Indeed, this may often be the factor that determines the overall heat transfer coefficient of the exchanger. With the HF fiber, however, a great variety of inner diameters are easily achieved, often much smaller than is practical with metal tubes. The smaller the ID of the fiber, the larger the ratio of heat transfer area to volume of fluid within it. For example, in a 1.3 mm ID fiber 1 meter long, the heat transfer area is 40.8 cm², and the volume of water within it 0.13 mL, an area to volume ratio of 30.8. In a metal tube of ½″ ID, 1 meter long, the area is 398.9 cm², and the volume of water within the tube is 126.7 mL, a ratio of area to volume of 3.2. The smaller diameter HF has nearly 10 times more area in contact with a laminar boundary fluid. This can give a significant advantage to HFHX over shell and tube metal HX in liquid/liquid heat transfer applications, as laminar flow is often expected in pipes of the diameters typically used in shell and tube heat exchangers.

Another advantage of a smaller diameter HFHX is in the foot-print of such a device, compared to conventional metal heat exchangers. The high area to volume relationship of the HFHX means that much larger heat exchange areas may be put into a given volume of heat exchanger. For example, an 8″ diameter shell and tube heat exchanger, 1 meter long, with a tube packing density of 35%, may hold approximately 13 m² of heat exchange area in the interior of the tubes. In the same footprint, a HFHX with 1.3 mm ID fibers, at a packing density of 35%, can hold approximately 90 m² area in the interior of the fibers, or approximately 7 times the heat exchange area for the same footprint.

An characteristic of asymmetric (as used herein, a fiber with a dense non-porous layer over a region with small pores, which gradually become larger pores towards the opposite side of the fiber) polymer hollow fibers is that the void space in the pores of the fiber may be filled with materials other than the polymer itself. In one example, this void space could be filled with air or other gas. In this example, the overall heat transfer performance of the fiber would be reduced compared to a dense, non-porous fiber or tube, as air is an insulator relative to the polymer. For this reason, air spaces in the porous structure of the fiber should be avoided, for example, by wetting out the fiber, for example by soaking in an ethanol solution, before use. Once wetted, the void space may be filled by water. The thermal conductivity of water (˜0.6 WmK) is slightly above that of a polymer such as polyimide (˜0.52 WmK), so a water filled pore asymmetric polyimide HF would conduct heat as well or slightly better than a dense, non-porous tube of polymer, which would require substantially more polymer material to form.

In another example, the pores of an asymmetric hollow fiber HX in many water treatment applications may scale with sparingly soluble salts, such as calcium carbonate, calcium or magnesium sulfate, silica, and the like. These salts would scale within the porous structure of the HF, eventually filling the void space. Although the lumen of the fibers might be kept clear of scale with acid treatment, the porous structure will likely be resistant to such cleaning. The resulting fiber, however, is in many respects improved over one in which the void space is filled with water. For example, the thermal conductivity of silica is approximately 1.4 WmK, almost 3× higher than the polyimide polymer forming the fiber, and over 2× higher than water. The silica may also be expected to be substantially stronger with respect to mechanical robustness than many of the polymers that may be used. In fact, the benefits of a mineral scale filled porous polymer fiber are attractive enough to consider filling the pores of the fiber with such substances intentionally, as part of the manufacturing process. Such a fiber may be considered to be a polymer scaffold with a mineral filler, a combined material that will be stronger, better at conducting heat, and less expensive than a dense tube formed from polymer alone. This technique differs considerably from hollow fibers formed with a polymer solution containing similar materials prior to fabrication—the method contemplated herein applies these materials to the porous void spaces after the fiber has been formed, and not to the polymer itself.

Similar considerations apply to the formation of a scaling layer on the outer surface of the HF. In metal heat exchangers, the formation of such a scaling layer typically reduces heat exchange performance, as the heat transfer coefficient of the scale is substantially less than that of the metal. In the HFHX, the scaling layer is often a better thermal conductor than the polymer, which changes the relationship between scaling and heat exchanger performance. For example, a scaling layer of 1.4 WmK silica over the surface of a polymer fiber with a thermal conductivity of 0.52 WmK would not significantly decrease its performance, even if a thickness of such a material was formed that equaled the thickness of the fiber itself. For example, a 100 micron thick layer of silica (a quite thick scaling layer) over a 100 micron thick polymer fiber (each with the conductivities shown above), would have heat transfer coefficients of 5,200 Wm²K (fiber) and 14,000 Wm²K (silica). Since heat transfer rates are dominated by the highest component of resistance, in this case the lowest conductivity, the scaled fiber would conduct heat at nearly the same rate as one that was without scale. Scaling would then be detrimental only to the pressure drop of water flow through the exchanger, rather than to the heat transfer of the HX itself. This equates to a more robust HFHX performance with respect to scaling phenomena than is currently encountered in metal HX. In an embodiment, intentional scaling of the surfaces of the fibers during the manufacturing process may enhance their chemical resistance, mechanical strength, resistance to abrasion, and the like, without reducing their heat transfer performance.

The improvement in HFHX robustness and performance provided by mineral scales of the type commonly encountered in natural waters, as well as to other materials that may be deposited in and/or on the fibers to achieve similar or superior effects, are desirable enough to add them intentionally, as has been described, but also desirable enough to protect these coatings and/or fillings once applied. In an embodiment, a polymer hollow fiber for heat exchange that has been filled and/or coated with a mineral or other substance that may be soluble in water or otherwise subject to removal during HX service, may be protected by the addition of a thin layer of another substance to prevent its loss. Non-limiting examples of such tertiary coatings include siloxanes, epoxies, vapor deposition of films, thermal applications of melted polymers, and the like. The resulting three component fiber is stronger, more reliable, and of higher performance than a polymer HFHX alone.

Consideration of other differences between the polymer and metal materials show that many of the differences are in fact to the benefit of the polymer. A polymer heat exchanger is not vulnerable to pitting or corrosion cracking from chlorides in water, a common vulnerability of stainless steel, for example. Preferably, the polymer film is thin enough to limit heat transfer resistance, but robust enough to be durable in the presence of pressure, shear forces, and other causes of wear and tear, through many years of service life. Preferably, this is accomplished while using as little expensive polymer material as possible, and an inexpensive manufacturing method that allows for a minimization of heat exchanger cost. Embodiments below include methods of fabrication of phase inversion asymmetric polymer fibers suitable for such applications.

In accordance with one or more non-limiting embodiments, a polymeric hollow fiber may be formed for use in heat transfer applications. Such fibers may be placed into modules in manners similar to those used in the fabrication of hollow fiber or tubular membrane modules used for microfiltration, ultrafiltration, or reverse osmosis, for example. Heat transfer applications using such fibers, particularly in environments with saline waters, relatively low temperature and pressure environments, and/or otherwise corrosive or chemically challenging fluids, benefit from the use of such fibers. Benefits may include enhanced chemical and corrosion resistance, high area to volume characteristics, and reduced cost, without sacrificing heat transfer performance relative to that cost.

In an embodiment, a solution of polymer with desirable heat exchange characteristics may be formed by dissolving the polymer in one or more of several suitable solvents. These solvents may include, by way of non limiting example, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), Dimethylacetamide (DMAc), N-Methyl-2-pyrrolidone (NMP), acetone, tetrahydrofuran (THF), hexane, toluene, orthodichlorobenzene (ODCB), 1,2,3-trichlorobenzene (TCB), hexafluoroisopropanol (HFIP), m-Cresol, and the like. In some cases, solubility in the solvent may be enhanced by the pretreatment with or introduction of strong acids, bases, or other reagents to the solution, as well as in some cases by heating of the polymer and solvent. Polymers may include, by way of non-limiting example, polyimide, polyether ether ketone (PEEK), polyvinylidene fluoride (PVDF), polysulfone, polyethylene (PE), polyphenylene sulfide (PPS), polyphenylsulfone (PPSF or PPSU), poly(p-phenylene oxide) (PPO), polysulfone (PSU), high temperature nylon (HTN), cross-linked polyethylene (PEX), polytetrafluoroethylene (PTFE), Kynar®, Nylon, and polypropylene (PP). The polymer solution may be formed such that it contains a volatile solvent, such as, by way of non-limiting example, acetone, THF, or hexane. This solution is directed to a spinneret or similar device in which a bore fluid is directed to flow through a tube which is surrounded by a larger tube through which the polymer solution flows. Bore fluids, by way of non-limiting example, may be water, solvents, a mixture of water and solvents, air, nitrogen or other inert gas, and the like. Configurations of bore fluid and polymer solution outlets within the spinneret may vary with respect to the dimensions of each pathway and the number and geometric arrangement of these pathways within the polymer/bore fluid delivery device, to include the formation of multiple fibers per device. The polymer solution and bore fluid are co-extruded from the spinnerets or similar device over a precipitation fluid, which, by way of non-limiting example, may be comprise water, glycerol, alcohol, or one of many other fluids that are known to cause the precipitation of the various polymers from solution upon mixing with the bath fluid. The polymer and bore streams are caused to flow through a space above the bath fluid, often called an “air gap”, during which time evaporation of a portion of the polymer solvent occurs, causing the formation of a dense skin of polymer at the surface of the fiber. In some embodiments, this process may be enhanced by the use of a convective air flow air, inert gas, or the like, to accelerate or otherwise facilitate the formation of the skin layer. The fiber then enters the precipitation bath, causing a “de-mixing” of the polymer from the solvent solution as the solution and bath fluid, which is typically miscible to some degree with the solvent, mix, decreasing the solubility of the polymer, as well as possibly a change in temperature that assists in the formation of the desired structure of the fiber. This forms a porous polymer layer beneath the skin of the fiber that formed by the evaporative and/or thermal process. The resulting fiber may be further post-treated to increase cross-linking of the polymer, by way of non-limiting example, heating, exposure to UV light, or chemical treatment with acids, bases, reagents, and the like. The resulting fiber is suitable for heat exchange. The thickness of the fiber may, by way of non-limiting example, be in the range of 25-500 microns, preferably in the range of 75-250 microns. The diameter of the fiber, by way of non-limiting example, may be between 0.5-25.4 mm, preferably being in the range of 1-13 mm.

In an alternate embodiment, the fiber is exposed to a soluble substance that may be induced to form a solid within the porous structure of the fiber, and/or on its surface. This may include, by way of non-limiting example, silica, calcium carbonate, calcium sulfate, magnesium sulfate, organic colloids, inorganic colloids, sol-gel solutions, or other materials that may precipitate or otherwise deposit around and/or within the asymmetric hollow fiber. Precipitation may be achieved, by way of non-limiting example, by heating fiber that has been soaked in a solution of salts that precipitate when heated; illuminating with a UV light fiber that has been soaked in a solution of UV polymerizable components, such as colloids, monomers, or the like; drying the fiber after having been soaked in a solution of the desired filling and/or coating material. Other techniques of filling and/or coating the fiber may be employed, depending on the characteristics of the desired filling and/or coating material. Characteristics of the filling and/or coating material, by way of non-limiting example, may include a hydrophilic or hydrophobic character, chemical resistance characteristics, desired smoothness or roughness, mechanical strength, and/or other characteristics that may enhance the heat transfer performance, robustness, cost, or other desired characteristics of the HXHF and/or its module.

In an embodiment, the heat exchange fiber is fabricated into a heat exchanger module, by means commonly employed in the fabrication of hollow fiber modules for use in microfiltration (MF), ultrafiltration (UF), or reverse osmosis (RO) applications. These techniques include, by way of non-limiting example, the cutting of the fibers into lengths, by way of non-limiting example, between 0.5 and 3 meters, with both ends placed in epoxy, which once hardened, is partially cut away, exposing the openings of the fibers, while preventing the mixing of fluids that flow within the fibers from those which flow on the outer surface of the fibers. The configuration of such modules may be seen to be similar in many respects to those of shell and tube heat exchangers made from metal materials, but may also be suspended without shells directly into a fluid, as is commonly done in membrane bioreactors with MF or UF fiber arrays.

In another embodiment, the fiber pores are fully wet prior to use or treatment with filling and/or coating material. Some techniques to achieve this, by way of non-limiting examples, is to soak the fiber in a solution of ethanol, surfactant, glycol, or other substance that causes changes in the surface tension of the polymer to ensure its wetting. The fibers may subsequently be coated, on either or both sides and within its porous structure, with a substance to enhance its hydrophilicity and tendency to remain wet while being transferred, installed, and during service. One non-limiting example of such a substance is polydopamine

In an alternate embodiment, the application of mineral or other filling and/or coating material may be carried out on the fiber after it has been fabricated into a module format.

In an alternate embodiment, no air gap is used in the formation of the fiber. Rather, a skin is formed based on the selection of the precipitation bath fluid and its characteristics. These characteristics, by way of non-limiting example, may include temperature, pH, reactants, and other substances or attributes that may be expected to cause the formation of a pore free skin of the desired thickness on the fiber surface. One non-limiting example of a non-evaporation based skin formation is the use of a bath fluid that is only partially miscible with the polymer solvent, tending to form a skin at the interface of the two upon immersion in the precipitation bath. In an alternate embodiment, the concentration of polymer in the solution is high enough to induce a dense, non-porous skin to form upon immersion in the precipitation bath, whether or not an evaporation step was employed.

In an alternate embodiment, the skin is caused to form on the interior of the fiber, and the porous structure on the outer part of the fiber.

In an alternate embodiment, a skin is formed on both the inner and outer sides of the fiber. In some cases, one of these may be expected to contain some remaining pores, although the skins may be substantially non-porous, even though the interior of the fiber between the skins may be remain porous.

In an alternate embodiment, the selection of solvent and configuration of the air gap of the fiber formation causes a substantial portion of the fiber to become dense, with little to none of the fiber to be porous, as most or all of the precipitation is caused by the evaporation process. Any remaining pore voids may, in some embodiments, be filled with a secondary material.

In an alternate embodiment, a HFHX module containing fiber that has not been coated and/or filled with another material is fully wetted and placed into service. Over time, scaling potential in the fluids passing within the HFHX module cause deposition of mineral scale into the porous structure of the fibers and/or onto its surface. The resulting fiber is periodically cleaned by, for example acid treatment, but remains substantially filled and/or coated with mineral scale and is substantially unaffected in heat transfer performance, and may be improved in mechanical strength, abrasion resistance, and chemical resistance, as well as other characteristics. The scaled HFHX system in this embodiment is then considered the matured or finished product.

In an alternate embodiment, the HXHF module is used in a falling film heat transfer application, in which a thin film of one fluid is caused to flow on the surface of the fiber, either inside or outside, while heat is transferred to or from a second fluid on the other side of the fiber. In some embodiments, a third fluid may be used to transfer heat and/or mass to or from the first fluid. The first and second fluids may be liquids, for example. The third fluid may be an immiscible liquid or a gas, for example. The module may be substantially or fully vertical. It may have an integrated distributor for the fluids to ensure proper formation of a film. It may have a coating and fill material that causes it to have a hydrophilic surface, enhancing its tendency to form a film. One side may be hydrophobic, to enhance heat transfer from steam condensation, for example, and the other hydrophilic, to enhance the formation of an aqueous film.

In an alternate embodiment, a film of a first fluid is caused to flow on one surface of a HXHF, while a second fluid is caused to flow in a film on the other side of the fiber. Heat is transferred between the first and second fluids. A third fluid is used to transfer heat and or mass to or from the first fluid, and a fourth fluid is used to transfer heat and or mass to or from the second fluid. The third and fourth fluids may be immiscible liquids or gases, for example.

In an alternate embodiment, the HX fibers may be incorporated into a heat transfer device that differs in form from a shell and tube or filtration membrane format. By way of non-limiting example, this may include a multi-effect evaporator or similar device. One or more of the fluids involved in the heat or heat and mass transfer may be sprayed or otherwise distributed on a bundle or other array of HX fibers.

In an alternate embodiment, the filling and/or coating substance may be chosen so that in the course of normal cleaning during its use, conditions for the removal of this material are not expected to be encountered. By way of non-limiting example, the coating may not be soluble in the pH ranges used to normally remove scaling in a heat transfer application, such that although the scale is removed, the coating and filling material is not. In another embodiment, the material undergoes a change that reduces its solubility once precipitated, for example, by cross-linking, annealing, or other method, so that the material does not require protection from dissolution to remain in place.

In an embodiment, fully imidized polyimide polymer is dissolved in a solution of DMF and THF. The dissolved imidized polyimide is then extruded through a spinneret with a bore fluid including water, through an air gap of between 6-18 inches to induce the formation of a skin on the surface of the fiber as THF evaporates from the solution. The fiber is then immersed in the precipitation bath, which comprises water. The immersion causes the remainder of the polymer to precipitate, forming the HXHF. The fiber is then taken up on a drum and removed for potting. Following drying, the fiber is potted into a shell and tube style module with a two part epoxy with temperature and chemical tolerance compatible with that of the fiber polymer. This module, once cured, is subjected to a flow of a concentrated solution of calcium carbonate and heated, causing solid CaCO₃ to form in the pores and on the surface of the fiber. The resulting HXHF module is ready for service.

In an alternate embodiment, asymmetric HX hollow fibers are cast, formed into a module, and treated to fill their pores and/or coat their surface with a material that increases the physical strength of the fiber without significantly decreasing the fiber's heat transfer performance. The fiber is then coated with an additional, secondary material, on one or both sides, to protect the filling and/or coating material from wear or dissolution. By way of non-limiting example, the filling and/or coating material may be silica, calcium carbonate, calcium sulfate, magnesium sulfate, organic colloids, inorganic colloids, sol-gel solutions, or other materials that may precipitate or otherwise deposit around and/or within the asymmetric hollow fiber. By way of non-limiting example, the secondary coating material may be PDMS, other siloxane, epoxy, latex, PTFE, or any material that may be coated in a thin layer on the HF inner and/or outer surface that may be expected to protect the filling and/or coating material and/or the fiber polymer during service. These may include materials which are cured by heat, UV, drying, etc, in some cases with the addition of a cross linking agent.

In an alternate embodiment, the secondary coating may be introduced prior to the formation of the HX module.

In an embodiment, a HXHF is formed from high temperature polyimide, formed into a module, filled with calcium carbonate by thermal precipitation, and coated on the porous side with a thin coating of PDMS (for example, 5-10 microns). One non-limiting example of the PDMS coating method is the use of a dilute PDMS solution in hexane with an appropriate concentration of curing agent, followed by a curing step, which in some cases will involve heating and drying the fibers. In an alternate embodiment, the PDMS may subsequently be coated with a substance to enhance its hydrophilicity and ability to form thin aqueous films. One non-limiting example of such a substance is polydopamine.

In an alternate embodiment, materials may be added to the polymer solution to enhance the strength of the fibers. These may include, by way of non-limiting example, carbon fibers, nanotubes, glass fibers, high temperature nylon fibers, Kevlar or similar fibers, or strands of high strength, temperature resistant polymers that are not soluble in the organic solvents employed.

In a preferred embodiment, carbon fibers are mixed into a solution of PVDF in DMF and THF, cast into hollow fibers by the dry/wet method, and potted in a module using high temperature epoxy. The fiber pores are filled with soluble silica that is induced to precipitate within the pores by chemical reaction. Both sides of the fiber are coated with a thin layer of solvent free epoxy, such as that typically used to coat the interior of pipes. The fibers are subsequently treated with polydopamine to increase their hydrophilicity. In alternate embodiments, other types of coatings may be used including, by way of non-limiting example, solvent transported polymers, silicones or other siloxanes, thermally set epoxies, urethanes, solvent or gas carried films such as epoxies, SiO₂, TiN, or melt coatings of polymers such as PTFE. A heat exchanger fiber of this type may be thought of as a fiber-reinforced polymer scaffold with mineral filled pores and epoxy coating. Such a mixed-matrix heat transfer material offers high performance characteristics, particularly in heat transfer applications where the boundary conditions of the fluids are limiting, and low cost relative to performance.

FIG. 1 is a schematic illustration of an asymmetric polymeric hollow fiber 100 according to an embodiment. The asymmetric polymeric hollow fiber 100 includes a first, substantially impermeable portion 102 and a second, porous portion 104. As illustrated, the first, substantially impermeable portion 102 forms an outer skin of the asymmetric polymeric hollow fiber 100. The second, porous portion 104 forms an inner layer of the asymmetric polymeric hollow fiber 100. The outermost surface of the first, substantially impermeable portion 102 forms an outer surface 108 of the fiber 100. The innermost surface of the second, porous portion 104 forms an inner surface 106 of the fiber 100.

In an embodiment as discussed above, a soluble substance may be located in pores of the porous portion 104. As discussed above, the soluble substance may include, by way of non-limiting example, an inorganic material (e.g., an inorganic oxide, carbonate or sulfate, such as silica, calcium carbonate, calcium sulfate, magnesium sulfate, etc.,), organic colloids, inorganic colloids, sol-gel solutions, or other materials that may precipitate or otherwise deposit around and/or within the asymmetric hollow fiber. In an embodiment, at least one of the inner surface 106 and the outer surface 108 may comprise a hydrophilic coating or a hydrophobic coating.

FIG. 2 is a schematic illustration of an asymmetric polymeric hollow fiber device 200 according to an embodiment. The device 200 may comprise a plurality of asymmetric polymeric hollow fibers 100 made by any of the methods described above. In an embodiment, the device 100 includes a first support 206 and a second support 208. First ends of the plurality of asymmetric polymeric hollow fibers 100 are located in the first support 208 and second, opposing ends of the plurality of asymmetric polymeric hollow fibers 100 are located in the second support 206. The device 100 may be configured such that a first fluid 202 flows through the inside of the plurality of asymmetric polymeric hollow fibers 100 while a second fluid 204 flows around the outside of the plurality of asymmetric polymeric hollow fibers 100. In an embodiment, the device 100 is a heat transfer device which operates as discussed above.

FIG. 3 is a schematic illustration of a method of using an asymmetric polymeric hollow fiber 300 according to an embodiment. In this embodiment, the asymmetric polymeric hollow fiber 100 is configured such that a first fluid 306 flows inside the fiber 100 and a second fluid 308 flows along the outside of the fiber 100. In this embodiment, a third fluid 302 may be provided inside the fiber 100 and a second fluid 304 around the outside of the fiber 100. As illustrated, the first fluid 306 and the third fluid 302 are configured to flow countercurrently and the second fluid 308 and the fourth fluid 304 are also configured to flow countercurrently. Alternatively, the first fluid flow 306 and the third fluid flow 302 may be configured to flow concurrently. Additionally, the second fluid flow 308 in the fourth fluid 304 may be configured to flow concurrently.

In an embodiment, the first fluid 306 may be a liquid that is attracted to the inner wall of the asymmetric polymeric hollow fiber 300. Optionally, the inner wall of the asymmetric polymeric hollow fiber 300 may be coated with a hydrophilic coating to increase the attraction to the first fluid 306. The second fluid 308 may be a liquid that is attracted to the outer wall of the asymmetric polymeric hollow fiber 300. Optionally, the outer wall of the asymmetric polymeric hollow fiber 300 may be coated with a hydrophilic coating to increase the attraction to the second fluid 308. The third fluid 302 and the fourth fluid 304 may be gases that are immiscible with the liquids (first fluid 306 and second fluid 308, respectively) and which may carry away evaporated components from the liquids.

Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety. 

1. A method of making an asymmetric polymeric hollow fiber comprising: dissolving a polymer in one or more solvents to form a polymer solution; providing the polymer solution to a spinneret or device in which a bore fluid is directed to flow through a first tube surrounded by a larger, second tube through which the polymer solution flows; co-extruding the polymer solution and bore fluid; forming a first, substantially impermeable portion of the asymmetric polymeric hollow fiber; and forming a second, porous portion of the asymmetric polymeric hollow fiber.
 2. The method of claim 1, wherein the substantially impermeable portion comprises a polymer skin layer of a polymer fiber formed by evaporating a portion of the one or more solvents.
 3. The method of claim 3, wherein the porous portion is formed by directing the polymer fiber to a precipitation fluid, the precipitation fluid extracting solvent from an inner portion of the polymer fiber.
 4. The method of claim 1, wherein the substantially impermeable portion is formed by reaction in the precipitation fluid.
 5. The method of claim 1, further comprising pretreating the polymer prior to dissolving the polymer to enhance the solubility of the polymer in the one or more solvents. 6-11. (canceled)
 12. The method of claim 1, further comprising post-treating the asymmetric polymeric hollow fiber after forming the porous portion to increase cross-linking of the asymmetric polymeric hollow fiber by heating, exposure to UV light, or chemical treatment with acid, base or reagent. 13-16. (canceled)
 17. The method of claim 1, further comprising exposing the asymmetric polymeric hollow fiber to a substance which forms a solid deposit within pores of the porous inner polymer layer. 18-23. (canceled)
 24. The method of claim 1, wherein the first, substantially impermeable portion is located on an outside surface of the asymmetric polymeric hollow fiber, the second substantially impermeable portion is located on an inside surface of the asymmetric polymeric hollow fiber and the porous portion is located between the outside and inside surfaces of the asymmetric polymeric hollow fiber.
 25. A method of making heat transfer device comprising the asymmetric polymeric hollow fibers of claim 1, comprising: cutting the asymmetric polymeric hollow fibers into predetermined lengths, each fiber having a first end and an opposite second end; mounting the first ends in a first mount and the second ends in second mount; and removing portions of the first and second mounts, thereby exposing openings of the asymmetric polymeric hollow fibers. 26-29. (canceled)
 30. A method of using an asymmetric polymeric hollow fiber device having at least one asymmetric polymeric hollow fiber comprising an impermeable surface and an opposite porous surface, the method comprising: providing a first fluid having a first temperature to an inner surface of a wall of the asymmetric polymeric hollow fiber; providing a second fluid having a second temperature to an outer surface of the wall of the asymmetric polymeric hollow fiber, wherein the second temperature is different from the first temperature; and transferring heat from the first fluid to the second fluid or the second fluid to the first fluid through the wall of the asymmetric polymeric hollow fiber.
 31. The method of claim 30, further comprising providing a third fluid, the third fluid configured to transfer at least one of mass or heat to or from the first fluid.
 32. The method of claim 31, further comprising providing a fourth fluid, the fourth fluid configured to transfer at least one of mass or heat to or from the second fluid.
 33. The method of claim 30, wherein at least one of the inner surface and the outer surface comprise a hydrophilic coating or a hydrophobic coating.
 34. An asymmetric polymeric hollow fiber comprising: a first, substantially impermeable portion; and second, porous portion.
 35. The fiber of claim 34, wherein the impermeable portion comprises an outer skin of the fiber.
 36. The fiber of claim 35, further comprising a second impermeable portion comprising an inner skin of the fiber, the second porous portion located between the first and second impermeable portions.
 37. The fiber of claim 36, further comprising a solid deposit located in pores of the porous portion.
 38. The fiber of claim 37, wherein the asymmetric polymeric hollow fiber comprises an inner surface and an outer surface and at least one of the inner surface and the outer surface comprise a hydrophilic coating or a hydrophobic coating.
 39. An asymmetric polymeric hollow fiber device comprising a plurality of asymmetric polymeric hollow fibers of claim
 34. 40-43. (canceled) 